Vertically stackable tray sets are well known, and one of the most common applications of such sets is for use as multi-level in and out boxes which sit atop an office desk. Such tray sets in the past have included an upper tray, a lower tray, and a tray connector which is secured in some fashion to the bottom of the upper tray and the top of the lower tray. The function of the connector is to keep the two trays in assembled relationship with each other. In addition, the connector often serves the function of spacing the two trays vertically from each other so that the amount of material which can be stored in the vertical space between the two trays is increased. An example of such a tray set is seen in Juergens U.S. Pat. No. 4,074,810.
It is also known that the trays may be formed with double walls having interior spaces for receiving one or more bayonet members formed at the upper and lower ends respectively of the connector/spacer member. These bayonets are plugged into the space between the tray walls, entering through upper and lower ports formed in the walls for that purpose. An example of a tray set constructed in this fashion is seen in Serwer U.S. Pat. No. 3,533,512.
One of the disadvantages of this type of construction is that the topmost tray of the vertical stack has an unsightly upwardly facing port opening which is designed to accommodate the connector bayonet for another tier of trays which, however, are not going to be used. It is desirable that some means be provided to close the upwardly facing port opening of the topmost tray. (In speaking of the "topmost tray" this reference is intended to include not only the uppermost tier of a multi-level stack, but is also intended to include a single tray which is not stacked at all.) Furthermore, it is desirable that the upwardly facing port not be closed off by an integrally molded plug which is designed to be broken out in order to permit insertion of the connector bayonet. If the plug must be broken out, then the user cannot thereafter change his or her mind, and restore the plug to its former position. As a result, if a particular tray is first used as a lower level tier of a stack, it cannot thereafter be converted to use as a topmost tray without displaying an unsightly upwardly facing port opening.